The invention pertains to a process for the control of a motorized feed drive for a movable workpiece table of a machine tool, especially the rolling carriage of a circular saw, and a device for the execution of the process, with a workpiece table that with the aid of a motorized feed drive is longitudinally movable.
On many machine tools, such as circular saws, the workpiece feed is done by hand. In the case of a circular saw, a workpiece to be sawed is laid down by the operator on the rolling carriage of the circular saw and subsequently pushed toward the circular saw blade in the direction of the cut. The rolling carriage serving as support for the workpiece is moved along with the workpiece (DE-PS 24 09 420). Therefore, when advancing the workpiece, the operator has to overcome not only the cutting force, but also the friction existing between the rolling carriage and the machine frame. Furthermore, the rolling carriage must, prior to each cutting process, first be accelerated together with the workpiece, and after each cutting process be decelerated and subsequently retrieved. Especially with high-quality, i.e. warpfree (rigid against torsion), rolling carriages, the acceleration work for the acceleration and braking of the substantial mass of the rolling carriage is not inconsiderable. Added to this is the (mass) inertia of the workpiece.
Consequently, where many cuts are made in quick succession, the operator must spend substantial energy repeatedly overcoming of the described forces. This results in a faster exhaustion of the operator and in a loss of productivity.
For the overcoming the forces opposing the workpiece feed, motor-driven rolling carriages are conceivable and are already in use for special application cases. For example, for cutting of so-called postforming sheets a constant feed rate is partially applied, which can easily be achieved with a motorized drive. A known design of such a drive consists of a hose that can be charged with compressed air, and on which a drive pulley (wheel)xe2x80x94attached to the rolling carriagexe2x80x94can roll off. If the air hose is blown up on one side with compressed air, the radially expanding air hose replaces the drive pulley and drives thereby the rolling carriage in the direction of the feed.
DE-OS 42 22 906 describes feed hydraulics for sawing machines that adjusts the feed with a constant power corresponding to a set value (set point). Furthermore, electrical drives for a constant feed rate are known. (Periodical xe2x80x9cPLASTverarbeiterxe2x80x9d October 1983, pages 1123, 1124).
A common disadvantage of the previously known motorized or hydraulic or pneumatic feed drives is their lack of flexibility. However, particularly the high flexibility of the machine tools that can be simply operated manually justifies the expensive employment of the personnel required for their operation.
Therefore, the object of this invention is the creation of a process for the control of a motorized feed drive, especially for a rolling carriage, that satisfies high requirements for the flexibility of the drive, especially regarding various feed rates, and the creation of a device for the execution of this process.
According to the invention, this object is achieved by a process where the manual force applied by the operator in direction of the feed is sensed and converted into a corresponding measured value, a set value is created from it for the feed rate of the rolling carriage, and from that a control signal is derived for the drive.
The required high flexibility of the feed drive results from the process according to the invention by using the manual force used by the operator for influencing the feed rate of the rolling carriage generated by a motor. A particular advantage of the solution provided by the invention is that it does not require a significant readaptation (retraining) by the operator, because the feed rate depends, also in entirely manually operated machines, such as, e.g., circular saws, directly on the manual force applied by the operator in the direction of feed. The process simultaneously allows a reduction of the manual force that needs to be applied by the operator, to a fraction of that which would be required if the operator would have to effect the feed entirely with his own power. According to the invention, the power of the operator is only required as an input parameter for the control of the feed rate and thereby, in the end, independently of which forces have to be actually applied for the feed.
An advantageous variant of the inventive process is characterized by that the cutting force is determined and considered in the generation of the set value for the feed rate. The consideration of the cutting force allows it, e.g., to reduce the feed rate, if the cutting force rises. The reduction of the feed rate may then be compensated for by an increased manual force of (applied by) the operator, so that the behavior of the motor-driven rolling carriage, controlled according to the invention, approximates that of one that is manually driven. Furthermore, it is also possible to stop the feed in case of an extremely increased cutting force, in order to prevent standstill of the saw blade.
To prevent the start of the feed drive in case of very small manual forces, e.g., those unintentionally applied, the set value of the feed rate is in a preferred variant set at zero as long as the applied manual force does not exceed a threshold value.
Preferably, the set value is created in dependence on the manual force in excess of the threshold value, so that it increases with increasing manual force and decreases with decreasing manual force. This corresponds to the behavior of a manually driven rolling carriage.
Furthermore, a process is preferred in which the set value is created such that it does not exceed an upper limit value, in order to not let the feed rate grow limitless and to prevent control fluctuations triggered by the rolling carriage temporarily escaping the operator so that the manual force of the operator diminishes, resulting in a stoppage of the rolling carriage, to be subsequently set in motion again by the operator.
To assure that the rolling carriage exhibits the expected behavior also in dependence on the cutting force, a process is preferred in which the set value is created in dependence on the cutting force such that it decreases with increasing cutting force and increases with decreasing cutting force. In this case, the set value, even if just based on the influence of the cutting force alone, should preferably not fall below the value zero.
Preferably, upon initiation by the operator it should be possible to create also a negative set value for the feed rate so that the rolling carriage can also be moved backward by motor power. A preferred variant of the process according to the invention is characterized by a set value for the feed rate or a value of a speed control or RPM-control for the feed drive that is imposed as a reference input value.
The inventive solution to the aforementioned task comprises a device where a drive is connected with a control mechanism for the formation of a control signal for the speed of the drive, and the control mechanism contains a manual-force sensor and a control signal generator connected to it. The process according to the invention can be executed with the aid of such a device.
In order to allow also the consideration of the cutting force in the control of the feed drive, the control mechanism in a preferred form of the device contains additionally a device for the determination of the actual cutting force, where the device is also connected to the control signal generator.
In order to facilitate a motorized backward movement of the rolling carriage, the control mechanism of the inventive device preferably contains an element for the presetting of a negative feed rate.
Furthermore, the device according to the invention is preferably equipped with a controller for the feed rate that is tied to the control signal generator. Such a device allows the use of a value for the feed rate that corresponds to the control signal generator as reference input value for the control of the feed rate.
Preferably, the controller is part of the control mechanism or, in an alternative form of execution, part of the feed drive.